1. Field of the Invention
The present invention relates to a hydraulic control device of a continuously variable transmission for a vehicle and is related to a technique of adjusting an oil pressure.
2. Description of the Related Art
In a vehicle including a continuously variable transmission having driving-side and driven-side pulleys and a belt wound around the both pulleys in a power transmission path between an engine acting as a power source for running and drive wheels, it is known that the driving-side pulley is provided with a driving-side hydraulic actuator for changing a groove width thereof while the driven-side pulley is provided with a driven-side hydraulic actuator for changing a groove width thereof, and a winding diameter (effective diameter) of the belt on each of the both pulleys is varied to make a shift in the continuously variable transmission in a hydraulic control device of a continuously variable transmission for a vehicle. For example, this corresponds to a hydraulic control device of a continuously variable transmission for a vehicle described in Japanese Laid-Open Patent Publication No. 2013-160379.
The hydraulic control device of the continuously variable transmission for the vehicle controls each of a normally-opened first electromagnetic valve disposed in a first oil passage between a hydraulic source and the driving-side hydraulic actuator and a normally-closed second electromagnetic valve disposed in a second oil passage between the hydraulic source and the driven-side hydraulic actuator, thereby properly maintaining a tension of a transmission belt wound around the driving-side pulley and the driven-side pulley and continuously varying the transmission. A third oil passage connected between the first oil passage and the second oil passage is provided with a pressure reducing means (an orifice) reducing a hydraulic oil pressure supplied from the first oil passage to the second oil passage and a normally-opened third electromagnetic valve. As a result, at the time of failure when the electromagnetic valves are all put into an OFF-state due to occurrence of a failure of an electric power source for hydraulic control, disconnection of a harness supplying a drive control signal from an electronic control device to the electromagnetic valves, or a failure of the electronic control device controlling the electromagnetic valves, the hydraulic oil from the hydraulic source is supplied through the first electromagnetic valve to a first hydraulic actuator, and the oil pressure of the hydraulic oil supplied to the first hydraulic actuator is reduced by the orifice and supplied through the third electromagnetic valve to the second oil passage and the driven-side hydraulic actuator, so as to ensure running of a vehicle at the time of failure (limp home running).
In the hydraulic control device of the continuously variable transmission for the vehicle of Japanese Laid-Open Patent Publication No. 2013-160379, the hydraulic oil pressure of the hydraulic source is supplied through the first and third electromagnetic valves put into an opened state to the driving-side hydraulic actuator and the driven-side hydraulic actuator at the time of failure when the electromagnetic valves are all turned off and, since the hydraulic oil pressure is reduced by the pressure reducing means and supplied to the second oil passage or a second hydraulic actuator, the second hydraulic actuator is prevented from being supplied with an excessive oil pressure at the time of failure. This eliminates the requirements for higher strength and improvement in ensuring rigidity of a fixed sheave and a movable sheave facing thereto and the increases in axial dimension and mass for ensuring the strength of the driving-side pulley and the driven-side pulley. However, since the third oil passage connected between the first oil passage and the second oil passage must be provided with the pressure reducing means (the orifice) reducing a hydraulic oil pressure supplied from the first oil passage to the second oil passage and the normally-opened third electromagnetic valve, a hydraulic circuit configuration is disadvantageously complicated so as to ensure running of a vehicle during failure.